76 TUK BOTTOM OF THE SEA. 



near Callao, the port of Lima, a considerable dike of 

 rolled stones parallel to the shore has been thrown up. 

 Its height in some parts is nearly 20 feet. On the 

 landward side it is steepest, and on the other descends 

 towards the sea with a gentle slope ; the rolled 

 stones of which it is composed are spherical or oval 

 in shape. To account for its existence, there is, at 

 some distance off, between Callao and Moro-Solar, a 

 cliff, which measures some 140 feet in height. Its 

 base is incessantly battered and eaten away by the 

 waves, and the debris is carried away northward 

 by the current. In this remarkable instance the 

 current transports the materials of which the dike is 

 constructed, and the waves build them up. 



Where a marine current passes over a level bottom 

 and comes to a gentle slope, its speed is a little ac- 

 celerated by reason of the confinement of its bed ; 

 but this increased speed is lost again when the ob- 

 stacle has been passed. The case is different if the 

 bottom of the sea, instead of becoming gradually 

 higher, rises abruptly. A wave is then produced, 

 which propagates itself under the water, and causes 

 what is known as a ground-swell. This wave aug- 

 ments as it approaches the shore, and, advancing 

 rapidly, breaks in a mass of foam which escapes from 

 beneath the liquid bulk. If the shore be steep, it hurls 

 itself against it, and throws an immense head of foam 



